Surface exposure cosmogenic nuclide dating
These secondary fast nucleons continue to produce cosmogenic nuclides in the atmosphere, hydrosphere & lithosphere by breaking apart target atoms through spallation interactions.Eventually, the particles have insufficient energy to cause spallation. The production of cosmogenic nuclides slows with depth in rock as the cosmic ray intensity flux becomes attenuated with depth.This drawing illustrates air showers from very high energy cosmic rays. When cosmic rays collide with atoms in our atmosphere, they cause a cascade of reactions – we call this the ‘cosmic ray cascade’.The first interaction is when the high energy particles collide with nuclei in the upper atmosphere. A spallation reaction is a nuclear reaction where a highly energetic nucleon (usually a secondary cosmic-ray neutron of energy) collides with a target nucleus.Cosmic rays (also called cosmic radiation) mainly comprise high energy nucleons (protons, neutrons and atomic nuclei).About 90% are hydrogen nuclei (a single proton with an atomic number of 1).As such, the cosmic ray flux at the equator is four times less than the flux at the poles.
Cosmic rays pass through our galaxy at close to the speed of light.The muons and gamma rays then decay to form electrons (e).The hadronic component comprises protons (p) and neutrons (n).The particles bounce about in the magnetic field of the remnant anomaly until they gain sufficient energy to escape the system, whereupon they become cosmic rays. It is now known that most cosmic rays are atomic nuclei.Most are hydrogen nuclei, some are helium nuclei, and the rest heavier elements.The cosmic ray intensity flux also varies with altitude.The secondary particle flux, formed after that first interaction, peaks at 15 km altitude.This causes the release of multiple particles (protons, neutrons and clusters).These particles cause a wave of secondary interactions and spallation reactions.Therefore, most cosmogenic nuclides are formed within the top few centimetres of a rock.For cosmogenic nuclide dating, we are mostly interested in just six isotopes.